This invention relates to a wing unit, in particular a spar box, for forming aerodynamically active surfaces of an aircraft, in particular airfoils, horizontal tail units or rudder units of a plane, comprising an upper shell and a lower shell.
In aircraft construction, spar boxes form the wing unit as such for receiving all substantial mechanical forces or loads occurring at aerodynamically active surfaces, in particular airfoils, rudder units, horizontal tail units, or the like.
Such spar boxes, e.g. for forming an airfoil, are formed amongst other things by a lower shell and an upper shell. The upper shell and the lower shell form the aerodynamically active surface as such, representing e.g. an airfoil section, a horizontal tail unit section, or a rudder unit section, or the like, of a plane. In addition, between the lower shell and the upper shell, a plurality of ribs are preferably uniformly spaced from each other, and arranged substantially in parallel to the longitudinal axis of the plane. Amongst other things, the ribs are for connecting the upper and lower shells as well as for further stiffening of the spar box. The ribs are respectively connected by their lower and upper rib feet to the corresponding inner surfaces of the lower shell and the upper shell, at least sectionwise. Substantially transversely to the longitudinal axis of the plane, and/or approximately in parallel to a front and/or rear edge of the airfoil, in addition, normally at least one spar extends between the upper and the lower shell. If an airfoil comprises for instance one front and one rear spar, then the front end regions of the ribs are at least sectionwise connected to the front spar, and the rear end regions of the ribs are at least sectionwise connected to the rear spar. Besides, the spars are connected at least sectionwise to the upper and lower shells. For further stabilizing the surfaces of the upper and lower shells, the latter in general further have so called stringers. The stringers are made as profiles that are preferably uniformly spaced from each other, arranged substantially transversely to a longitudinal axis of the plane and/or in parallel to a front or rear edge of an airfoil, at the inner surface of the upper and lower shell and connected thereto. The stringers can be formed e.g. as angle profiles or the like.
As a result of the almost all-round connection of the upper and lower shell, as well as of the ribs and spars to each other, a spar box thus constructed for forming an airfoil, a horizontal tail unit, a rudder unit, or the like, is capable of bearing high loads while having a comparatively low weight.
At the spar box of an airfoil, amongst other things, the landing gear, engine pods, takeoff and landing flaps, slats, spoilers and so called flaps are secured. In addition, an adequately sealed spar box can at the same time serve as a fuel tank for a plane. If the spar box is not completely sealed, e.g. tightly closing foil linings, so called liners, can be inserted into the spar box for receiving fuel or the like. However, regardless of its secondary features, the spar box main object is to introduce all forces acting on the airfoil through the airfoil root into the fuselage cell of the plane. Inversely, forces from the fuselage cell are introduced through the airfoil root into the spar boxes and thus e.g. into the airfoils.
Also, further aerodynamically active surfaces of an aircraft, e.g. horizontal tail units, rudder units, canard wings, and other control or tail surfaces have spar boxes for forming the load carrying structure.
In earlier spar boxes, all occurring forces are transmitted through the respective components of the spar box, in particular the upper shell, lower shell, spars, ribs, and stringers. For an airfoil, e.g. the upper and lower shells of the spar box substantially transmit the normal stresses resulting from the transverse force bending of the airfoil. In contrast, the ribs in the spar box are mainly loaded with transverse forces. Finally, the spars are mainly for transmitting shear stresses resulting from bending of the airfoil. This distribution of the different forces occurring inside an airfoil, horizontal tail unit, or rudder unit, over respectively different constructive elements has proven to be very effective for the lightweight structures required for aircraft construction.
Therefore, due to geometry and statics uniquely defined during constructive design, earlier spar boxes have preset bending and torsion under load.
By using fiber reinforced plastic materials it is possible to obtain, though in a limited way—due to their at least passively differentiating effect—a direction dependent bending/torsion coupling of a spar box, however, dynamic adaptability of the spar box to a plurality of different operating or loading states is not possible in this way.
In addition, in different loading states of a plane, e.g. respectively a defined bending/torsion coupling of the spar box shall be reached, in particular so as to improve the aerodynamic properties of the plane in a plurality of different operating states and/or to reduce the mechanical load of individual plane components.